A cigarette packing line normally comprises a manufacturing machine for producing the cigarettes; a filter assembly machine for applying filters to the cigarettes; a packing machine for producing soft or rigid packets of cigarettes; a cellophaning machine for applying an overwrapping of transparent plastic material to the packets of cigarettes; and a cartoning machine for producing cartons of packets of cigarettes.
A feed unit is interposed between the cellophaning machine and the cartoning machine to receive a succession of packets of cigarettes from an output of the cellophaning machine and transfer the succession of packets of cigarettes to an input of the cartoning machine. The feed unit often has a reject station located along the path of the packets of cigarettes to remove from the path any faulty packets of cigarettes detected by control stations on the cellophaning machine. Location of the reject station at the feed unit is usually advantageous on account of the considerable size of the reject station, which must also collect the rejected packets of cigarettes and is difficult to accommodate on the cellophaning machine.
Some known packing lines of the type described above are designed to transfer from the output of the cellophaning machine to the input of the cartoning machine a succession of packets of cigarettes arranged in two or more superimposed rows, so as to reduce the average travelling speed, and hence mechanical stress, of the packets of cigarettes.
When feeding packets of cigarettes arranged in two or more superimposed rows, rejection of a faulty packet of cigarettes travelling through the reject station calls for also rejecting the good packet/s stacked with it. This is due to the way in which known reject stations are built and operate, which does not permit removal from the stream of a single packet stacked with another.
The feed unit may also comprise a heat-shrink station for heat treating each packet of cigarettes. For each row of packets of cigarettes, the heat-shrink station comprises a respective channel, along which the row of packets of cigarettes travels in use, and which is bounded at the top and bottom by two slide surfaces equipped with electric heating elements. When a packet of cigarettes is pushed along the respective channel at the heat-shrink station, the major lateral walls of the packet of cigarettes inevitably slide along the heated slide surfaces, thus generating friction on the packet of cigarettes, which is a function of the pressure exerted on the packet by the slide surfaces. To avoid subjecting the packet of cigarettes to severe friction which might damage or even tear the sheet of overwrapping material, the slide surfaces are spaced far apart. Such a solution, however, reduces the effectiveness of the heat treatment and calls for using very long heat-shrink stations.